1. Field of the Invention
The present invention relates generally to automatic flight control systems for aircraft and more particularly to a system for helicopters for providing control of engine torque under conditions where the flight control system is demanding more power than the engine can safely supply.
2. Description of the Prior Art
Autopilot systems particularly adapted to use with helicopter flight control have been used as disclosed, for example, in the present assignee's U.S. Pat. No. 2,845,623 issued July 29, 1958 to G. E. Iddings and U.S. Pat. No. 4,109,886 issued Aug. 29, 1978 to E. R. Tribken, et al. The present system is concerned primarily with the control of the airspeed and the vertical path of the craft and hence involves the cyclic pitch control and the collective pitch control for commanding the pitch attitude of the craft and its vertical movement, respectively. Control of other axes are not herein addressed.
In many prior art autopilot systems, when it is desired to control both the airspeed and vertical path of a helicopter, the pitch axis is used to control airspeed modes while the collective axis is used to control vertical path modes. The aircraft may be caused to accelerate or decelerate by a change in pitch attitude, while changes in the collective control setting vary the vertical thrust of the rotor system, resulting in a direct increase or decrease in lift. However, an increase in collective demand will also result in an increase in the power being demanded of the aircraft's engines and transmission. Since operational upper limits are imposed by the manufacturer on the allowable output of the power plant, expressed as a developed torque limit, it is desired that neither manual nor autopilot inputs should command outputs in excess of these allowable limits.
It has been found that when flying at engine power settings near the maximum allowable limit, increases in collective setting in order to enter a climb or capture a desired altitude during a descent may cause the demands on the power plant to exceed the allowable limits. Thus, it has been the practice to monitor the developed engine torque so that the autopilot collective servo drive may be cut off if maximum torque is exceeded, or to actively monitor and manually limit the amount of torque which can be commanded. Unfortunately, this approach may adversely effect the vertical performance of the aircraft. For example, if the aircraft is being flown in the maximum power region while descending in an altitude preselect mode, the autopilot will be unable to arrest the descent and capture the desired altitude if either of the above techniques are used, either due to the fact of disabling the collective autopilot, or the inability to provide the required additional collective torque. Another approach has been to disable automatic collective control only at high speeds, thus flying vertical path modes with pitch axis control only, while keeping the collective torque setting constant. However, this approach has the undesirable result of permitting an aircraft to exceed its maximum allowable airspeed when a descent is commanded since the airspeed is no longer in a controlled loop mode. It is also clear that where the pilot is required to monitor engine torque instruments and override the autopilot in the event of an over-torque condition, particularly during changes in attitude, this will result in an excessive work load as well as a potentially hazardous condition.